Optical gain mediums are used extensively in optical devices such as lasers and optical amplifiers. While such mediums may be liquid or gaseous, solid state gain mediums are being increasingly utilized in such applications for a variety of reasons including more compact construction and ease of handling.
Heretofore, a solid state optical gain medium has typically been constructed of a single semiconductor material. However, the semiconductor materials typically used for such applications have short absorption depths for optical photons, which depths are generally in the order of 1 to 2 microns. This absorption depth is roughly the same as the wavelength of the photon energy being amplified in such optical converter gain mediums, resulting in a relatively high numerical aperture for optical access to the medium. In other words, light tends to diverge strongly therefrom, making it difficult to couple light into and out of the medium.
Another problem with bulk semiconductor gain mediums is that they absorb and radiate optical energy at nearly the same wavelength. Thus, some of the energy being radiated is also reabsorbed, raising the pump density requirement for optical gain.
As a result of the problems indicated above, existing bulk semiconductor optical gain mediums operate with relatively high losses and relatively low quantum efficiency.
Further, in order to obtain external mode matching, it is desirable that the gain medium provide a particular optical and gain profile. Heretofore, because of strong beam divergence from the medium, heavy, bulky and expensive external optical elements have been required to achieve such mode matching. Such external optical elements are undesirable in all applications, and are a particular problem in applications where weight and space are at a premium, such as in satellites.
Therefore, a need exists for an improved optical converter gain medium which, under optical pumping, provides optical gain with high optical uniformity, controlled gain and optical intensity profile, high quantum efficiency, low loss and low numerical aperture for easy optical access.